1. Field of the Invention
The present invention relates to an electronic distributing type ignition device for internal combustion engine, and more particularly to an electronic distributing type ignition device characterized by multilayered structure to mount power transistor into the ignition controller in an ignition coil with build-in ignition controller.
2. Description of the Related Art
An ignition controller in ignition device for internal combustion engine is used for control the primary current in an ignition coil intermittently. In a general ignition device, the ignition coil and the ignition control device are separated. However, an ignition coil with build-in ignition controller, in which an ignition controller having a power transistor and an ignition coil are formed in a unit, is recently developed. The ignition coil with build-in ignition controller of this kind is poor in thermal radiation owing to being mounted on the engine block to increase its temperature. Therefore, the ignition controller requires to pay a sufficient attention against high temperature, and it is necessary to take the materials and combination used into the greatest possible consideration. Especially, for the multilayered chip structure in a power transistor part, cracking is apt to take place in the solder material in the multilayer part when the temperature goes beyond its operating environment range, in particular, its operating temperature range. for this reason, the multilayered structure is generally formed with taking the difference between thermal expansion coefficients into consideration. However, from a cost decreasing point of view, it is forced to use a low cost material even when the thermal expansion coefficient is largely different. Or in some cases, distortion is caused in the member due to the temperature during soldering to make the controller low thermal resistant.
In other words, the multilayer structure of power transistor chip is determined through considering how efficiently the heat generated the power transistor using silicon chip, that is, a power source, is transferred, and how the thermal expansion difference due to the temperature difference in the operating environment is absorbed. For example, in a case where a molybdenum plate is used as an absorbing material for thermal expansion difference, when the molybdenum plate and a copper heat sink are fixed to each other with solder in advance, these members are heated at temperature of approximately 650.degree. C. to melt the solder. In this time, since the linear expansion coefficient of molybdenum is 5.5.times.10.sup.-6 /.degree.C. and the linear expansion coefficient of copper is 1.67.times.10.sup.-6 /.degree.C., the fixed portion is some times arched due to the difference between the both linear expansion coefficients or due to the fix strength of the fixing solder, or embrittlement is caused in addition to arching depending on the composition of the copper heat sink. In order to avoid these, the following countermeasures have been taken: controlling the fixing area between the molybdenum plate and the copper base to be fixed, employing another structure, for example, such as fixing between the molybdenum plate and an alumina plate or using a copper plate instead of the molybdenum plate.